1. Field of the Invention
The present invention generally relates to a chucking hub arrangement of a spindle motor used in a floppy disk drive.
2. Prior Art
Floppy disk drives of today are in the trend of thin thickness, high accuracy and low cost. Spindle motors used in these floppy disk drives are accordingly required to be thin in thickness, accurate in performance, and cheap in cost.
Hereinafter, a conventional chucking hub arrangement of a spindle motor used in a floppy disk drive will be explained. This conventional chucking hub arrangement is used for the floppy disk drive having a hub base with the height not larger than 4.5 mm from the upper surface of a copper foil of a printed circuit board.
FIG. 3 is a cross-sectional view, in half, showing a spindle motor having a conventional chucking hub arrangement. FIG. 4 is a plan view showing the spindle motor of FIG. 3, seen from a drive pin side. A magnetic distribution or magnetization pattern is also shown in FIG. 3.
In FIGS. 3 and 4, hub base 1 is securely fixed with a rotor frame 2. An index magnet 3a is securely fixed to a medium holding magnet 3 integrally or by using bond. Around hub base 1, medium holding magnet 3 is securely fixed to rotor frame 2 integrally by molding or by using bond.
Medium holding magnet 3 is magnetized from the direction of an arrow A, while index magnet 3a is magnetized from the direction of an arrow B as shown in FIG. 3. And, as a result of such magnetization, a predetermined magnetic distribution or magnetization pattern is obtained as shown in FIG. 4.
A magnetic field sensing element 4 is fixed on an upper surface of a printed circuit board 5, to detect the magnetic distribution or magnetization pattern of index magnet 3a. A leaf spring 6 is swingable about a support shaft 7 securely fixed to rotor frame 2. A drive pin 8 is mounted on one end of leaf spring 6. The other end of leaf spring 6 is connected via spring 9 to a hook 2a provided on rotor frame 2.
An operation of the above-described chucking hub arrangement will be explained hereinafter.
First, a metal hub (not shown) of a recording medium is captured or drawn on a predetermined position on the upper surface of hub base 1 by medium holding magnet 3. A reference hole formed on the metal hub is engaged with drive pin 8 under application of a side pressure by spring 9. Then, the recording medium (not shown) is rotated by actuating the motor.
A recording-start-reference position of the recording medium is determined by an index signal. This index signal is generated once a complete revolution of the recording medium by the cooperation of index magnet 3a and magnetic field sensing element 4.
However, the above-described conventional arrangement was not reliable due to its inherent structural drawback. According to the above-described conventional arrangement, index magnet 3a is securely fixed to medium holding magnet 3 integrally by molding or by using bond. Hence, magnetizing index magnet 3a from the direction of arrow B, after medium holding magnet 3 is magnetized from the direction of arrow A, is likely to adversely affect to the magnetic distribution or magnetization pattern already formed on medium holding magnet 3. Such disturbance of magnetization will lead to the failure in the chucking operation, since the recording medium cannot be held stably on hub base 1.
On the contrary, if medium holding magnet 3 is magnetized from the direction of arrow A after index magnet 3a is magnetized from the direction of arrow B, an adverse effect will be given to the magnetic distribution or magnetization pattern already formed on index magnet 3a. This disturbance of magnetization will result in the deterioration of the index signal in its stability and accuracy, possibly causing dislocation of the recording-start-reference position of the recording medium leading to a fatal deficiency derived from this dislocation.
Even if the above-described magnetizations from the directions of arrow A and arrow B are simultaneously performed, the magnetization applied on medium holding magnet 3 will interfere with the magnetization applied on index magnet 3a because their phases are offset from each other. Hence, the above-described chucking error or the dislocation of the recording-start-reference position of the recording medium will arise.
In addition, the above-described conventional arrangement requires to apply the side pressure to drive pin 8 by spring 9 in order to avoid the above-described chucking error and stably accomplish the chucking operation. This will necessarily increase the number of parts and also increase the steps in the assembling of a product, accompanied by the increase of costs.